Operators use different middlebox services or appliances, called inline services, such as Deep Packet Inspection (DPI), logging/metering/charging/advanced charging, Firewall, Intrusion Detection and Prevention (IDP), Network Address Translation (NAT), etc., to manage subscribers' traffic. These services have high requirements on throughput and packet inspection capabilities. They can be transparent or nontransparent to the end users. Inline services can be hosted on dedicated physical hardware or in virtual machines.
Service chaining may be required if the traffic needs to go through more than one inline service. Moreover, if more than one chain of services is possible, the operator needs to configure the networking infrastructure to direct the right traffic through the right inline service path.